Ion beams are transported through a mass analyzer during the process of mass separation within the analyzer. Time of flight (TOF) mass analyzers and electrostatic trap (EST) mass analyzers may direct the ions upon lengthy and/or complex beam trajectories, especially where high mass resolution is to be obtained by the analyzer. For example, high resolution multi-reflection TOF (MR-TOF) mass analyzers may have beam path lengths of several meters, or several tens of meters and recent designs do not utilise long, straight field-free regions, but instead ions follow complex curved or folded ion trajectories, in some cases whilst continuously in the presence of an analyzer field. Examples of such analyzers include multi-sector MR-TOF designs such as are described in U.S. Pat. No. 7,399,960, multi reflection mirror TOF designs as described in WO 2005/001878 and helical path TOF designs such as are described in U.S. Pat. No. 7,186,972. A long and/or complex beam path within such an analyzer can be difficult to maintain precisely. Variation, upon entry to the analyzer, in ion beam characteristics such as spatial position, trajectory and beam energy for example, can cause the ion beam to fail to complete the desired ion beam path within the analyzer, possibly causing it, or part of it, to be lost by collision with some elements of the analyzer structure on route. With a complex beam path and a complex analyzer structure, failure to detect an emerging ion beam may render the analyzer unusable, as determining the cause of the problem can be too difficult to resolve. Alternatively, the ion beam may successfully traverse the analyzer, but may not follow the optimum beam path, travelling in regions where the analyzer fields are not as precisely maintained, thereby suffering beam aberrations. Interfacing preceding ion optical devices to such mass analyzers and tuning mass analyzers for routine operation may therefore be problematical.
Some designs of TOF and EST mass analyzers utilise electric fields, in some cases strong electrostatic fields, which are present along most or all the ion beam path within the analyzer, and for high mass resolution to be achieved by the analyzer, these electrostatic fields have to be precisely generated and maintained throughout the time the ion beam is within the analyzer. Where strong fields are present, yet higher precision is required of the injected beam characteristics as slight misalignments of the ion beam path are exaggerated by the strong field and the beam rapidly diverges from the ideal beam path. This problem is exacerbated when the flight path within the mass analyzer comprises multiple changes of direction.
In some forms of TOF mass analyzers, such as multi-turn analyzers, a single turn of the analyzer, followed by ejection to a detector outside the analyzer may be utilized to monitor the beam. A similar approach may be taken in other forms of mass analyzer where the beam path is folded back upon itself, i.e. ejection to an external detector for monitoring after the beam has traversed only a portion of the total beam path that would be used for a full mass analysis. In both cases an external detector is used for the full mass analysis at the end of the full flight path and the ion beam may be diverted from the main flight path to impinge upon it after any integer number of passes. However, for other types of analyzers the geometry of the analyzer and the resultant beam path may not conveniently allow ejection to the final detector until the entire flight path has been travelled. Monitoring the beam before it has followed the complete flight path is advantageous, as it allows the beam position and/or trajectory to be determined before multiple passes or multiple changes of direction have multiplied any beam misalignment, potentially causing the beam to be lost. It also allows a measurement of the quantity of ions within the beam in a much shorter time than would be required for a complete mass analysis involving multiple passes. However, multi-turn analyzers and other forms of mass analyzer where the beam path is folded back upon itself have limited mass range as low mass ions travel through the analyzer and catch up with high mass ions after multiple passes. The complex resultant spectrum of overlapping ions may be difficult or impossible to deconvolute and so a mass range restriction is required to avoid ions overlapping. It is desirable therefore to use mass analyzers in which no repeat path is used, in which case as already mentioned, the beam path may not conveniently allow ejection to the final detector until the entire flight path has been travelled.
It is also known in mass spectrometry to utilize parts of the analyzer structure as temporary beam monitoring devices. For example, the outer sector electrode of an electrostatic sector device may be disconnected from the voltage supply used to generate the electrostatic field during use as a TOF, and instead be connected to an electrometer. Ions, instead of being directed around the sector, collide with the outer sector electrode due to the absence of the analyzer electric field, and the current arriving at the outer sector electrode is measured by the electrometer. A similar approach may be taken utilising a lens within a MR-TOF, for example. However, use of analyzer electrodes as intermediate detection surfaces in this way renders the analyzer inoperable as a mass analyzer as the voltage supply must be disconnected for measurement to be made, and then reconnected again, and the analyzer field is thereby disrupted. The process is slow, and the measurement is made in the absence of the correct analyzer field. Beam aperture plates may be used as intermediate detector surfaces for beam monitoring, again using an electrometer. In the presence of the normal analyzer field, such an aperture plate transmits ion beam and the charge impinging upon the aperture plate is only a measure of the beam losses. In order to measure the charge present in the whole beam the analyzer field must be changed in order to direct the whole beam onto the aperture plate such that substantially no beam passes through the aperture. Furthermore, in recent multi-sector and MR-TOF designs, fields may be present throughout the analyzer and no such aperture plates may be present as they may both be unnecessary and would distort the electric field in their vicinity.
To measure smaller beam currents, secondary electron multipliers may be used, as are described for example by A. E. Giannakopulos et. al. in Int. J. Mass Spectrom. and Ion Processes, 131, (1994), 67. However these forms of detection system are costly, they require structures to be formed within the analyzer to house the multiplier, the presence of which may compromise the quality of the electric field within the analyzer. Furthermore, the detection efficiency of electron multipliers is dependent upon the chemical composition of, and strongly dependent upon the velocity of, the ions to be detected.
In view of the above, the present invention has been made.